spiral doesn't work. i tried it. imagine a square lawn, which most are, or at least rectangular. a spiral starts out great. you carve a big circle out of the square quickly. now you have 4 corners to mow and the remaining shape is such that you have some really difficult turns to make as you try to clear them. you end up having to go backward and forward in your tractor and it just sucks.

i found a modified zamboni approach to work very well.

Yes, I totally agree. I used to mow a mere 1.5 acres and it was all about minimizing turns of the 90 or 180 degree variety. I once tried to start in the middle and just mow a big spiral. It started out great except the mostly rectangular lawn then had some problematic corners to attend to and that was just a pain in the ass because if you wanted to clear a corner you suddenly had turns > 90 degrees and on my tractor that was really hard.

Being a hockey player I modeled my technique on what a zambonie does on the ice. the zamboni doesn't really like turns - it's on freaking ice- but it can do a 90 degree turn ok but never a 180. So it first goes down the middle and then across one end and up the far side, across the far end and up the middle again, but one-zamboni width over. Trying to guess the exact middle of your lawn is a bit tough and your efficiency largely rested on that initial choice.

At any rate, some overlap in lawn mowing is always preferably to never having to stop and turn around.